My next project is to try and save the small garage that is right next to my house. I have been told by a few people to tear it down and start from scratch, which I am sure might actually be easier and perhaps even cheaper (although I work for free and I occasionally can find a friend to help me, also for free)

I need to measure the building, in case anyone needs those measurements as they consider what advice to offer me.

Basically, the garage is out of square, portions of the foundation are cracked or settled, and the floor has gone Titanic on me. I actually am not worried about the floor, as
it is easy enough to pay someone to pour a new one, or I might even just pound it up and put gravel down for the floor, to save money.

I have read in a book about saving old barns that I might be able to jack up one wall at a time, pound out the damaged or settled sections of foundation, and then pour a pier every 6 to 10 feet, rather than trying to pour a whole new foundation wall. At the same time, I could replace the sill with some PT.

I've posted this question on another board I go to, but I've always thought that it is impossible to do too much research before you begin a project.

I'm eager to see if anyone has any advice. It looks sorry now, but I am hoping that with a few thousand dollars invested, I can get it back into shape!

Bull, when I was a pup, I had a garage just about like that. I put diagonal bracing on all four walls to steady it. Then I extended long boards across the narrow ends to give 4 jacking points, one on each corner. Using 4 jacks I just picked it up off the slab an inch or so, then broke out the old cement. I built concrete forms and poured a new floor over tamped sand. Set her back down and used it for years.

I actually enjoy digging holes and using a sledge...it's dumb labor, but I like the satisfaction of manual labor. Sometimes, I like it more than my teaching job!

How much concrete did you use for the new foundation? How did you mix it all? How large of an access trench did you dig to be able to access and work in the area? Did you dig trenches on the inside AND outside perimeters of the garage, or just on one side?

I actually enjoy digging holes and using a sledge...it's dumb labor, but I like the satisfaction of manual labor. Sometimes, I like it more than my teaching job!

How much concrete did you use for the new foundation? How did you mix it all? How large of an access trench did you dig to be able to access and work in the area? Did you dig trenches on the inside AND outside perimeters of the garage, or just on one side?

I have many more questions, but I'll stop for now.

If I did it again, I would rent an electric jack hammer to break up old cement.

It did not have a "foundation", it was just a thick slab, as I recall 4 to 6" thick. I calculated the amount of concrete by the volume of the forms. Length x width x thickness in cubic inches, convert to cubic yards. I then ordered it delivered in a concrete truck. Had to wheelbarrow most of it due to insufficient chute length on truck. As I recall (25 years ago) , there were no trenches, the form was just a flat slab.

Edit: Check your local building code. You may be required to have a foundation wall below frost level. In any case I think you can support the garage as indicated and break out and replace the slab / foundation.

Welcome, I love these projects.
Basically, first find if there is to much pest damage, termites or carpenter ants. This will save you from trying to save something which is already gone.
The next thing is to get a book on raising structures for your sanity when you ask questions. Basically so you can understand lingo ...most of the time we are as basic as possible. but this is a very big DIY project.
I personally have done many, all different.
The least expensive way is to do it in sections.
First you will need to support your frame. Start by cutting an 8 foot sill section out and install a 6x6 pressure treated new sill, on the existing foundation which is the at highest point possible. This will be your starting hieght. Then remove the foundation below and start fresh. You will need to install some angled supports up to the roof to help support the area you are working on.
I would reinstall the foundation with block rather than pouring foundation concrete. It will be more accessible and less work.
There is so much to tell you with this type of project, but a book can help and be there as you work . Then if some thing comes up....ask away.

This is not beyond someones capabilities...you just need an education on it first. Good luck, I know others will build on this.